where x is the wind stress, p^ the density of air, U the windspeed, 

 and Cj is a coefficient which must be evaluated from some combination 

 of theory and empirical data. The coefficient, Cj, depends on the ele- 

 vation at which the windspeed, U, is defined, the surface roughness, the 

 vertical temperature gradient in the air, the windspeed, and perhaps 

 other variables. Several proposed laws of Cj as a function of U are 

 shown in Figure 3. The variability of C^ is discussed in greater detail 

 in the next section. Well-designed laboratory experiments involving both 

 wind and water might be useful in obtaining a better definition of C^. 



7. Wind Stress on Floating Objects . 



Trajectories of floating objects, floats or drogues, are often used to 

 measure mean currents in a wave field. Since a part of the float must 

 be exposed to the wind, the resulting motion is determined partly by the 

 wind and partly by the water motion. 



Laboratory studies of floats and drogue motion in a water-wind facility 

 should lead to improvement in the interpretation of current measurements 

 obtained in this way. 



Wind plays a role in many other oceanographic phenomena of interest 

 to coastal engineers. The most important of the phenomena and a representa- 

 tive sample of those of secondary importance have been discussed in this 

 section to provide background for evaluating the technical discussion of 

 hydrodynamic phenomena in the following sections. 



III. MOMENTUM AND MECHANICAL ENERGY EXCHANGE BETWEEN AIR AND WATER 



1. Generation of Surface Waves . 



Modem studies of surface wave generation follow two basic lines of 

 development. The first, and simplest, is a heuristic development along 

 dimensional lines, with little consideration of microscale physical pro- 

 cesses. The second, more complex line of development, begins with a 

 consideration of the processes by which a. single water wave may gain energy 

 and momentum from the wind and seeks to explain the development of a wave 

 field by integrating, over all waves, the governing equations for a single 

 wave. The two approaches are not mutually exclusive and both require 

 empirical support from observations. 



a. Dimensional Analysis Applied to the Generation of Surface Waves . 

 It is readily verified from field observations that when an offshore wind 

 begins to blow, the wave height and period increase with distance from 

 shore and with the duration of the wind. Thus, the simplest realistic 

 model, which can be applied to wave generation near a well-determined 

 boundary after a substantial increase in windspeed, must depend on the 

 windspeed, the duration of the wind, and the fetch. The fetch is defined 

 as the overwater trajectory of the wind. Dimensional analysis shows 

 that the appropriate relations for wave height and period may be expressed 

 in the forms : 



14 



